doc/reftable: document how to handle windows
On Windows we can't delete or overwrite files opened by other processes. Here we
sketch how to handle this situation.
We propose to use a random element in the filename. It's possible to design an
alternate solution based on counters, but that would assign semantics to the
filenames that complicates implementation.
Signed-off-by: Han-Wen Nienhuys <hanwen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/technical/reftable.txt b/Documentation/technical/reftable.txt
index 8095ab2..3ef169a 100644
--- a/Documentation/technical/reftable.txt
+++ b/Documentation/technical/reftable.txt
@@ -872,17 +872,11 @@
Layout
^^^^^^
-A collection of reftable files are stored in the `$GIT_DIR/reftable/`
-directory:
-
-....
-00000001-00000001.log
-00000002-00000002.ref
-00000003-00000003.ref
-....
-
-where reftable files are named by a unique name such as produced by the
-function `${min_update_index}-${max_update_index}.ref`.
+A collection of reftable files are stored in the `$GIT_DIR/reftable/` directory.
+Their names should have a random element, such that each filename is globally
+unique; this helps avoid spurious failures on Windows, where open files cannot
+be removed or overwritten. It suggested to use
+`${min_update_index}-${max_update_index}-${random}.ref` as a naming convention.
Log-only files use the `.log` extension, while ref-only and mixed ref
and log files use `.ref`. extension.
@@ -893,9 +887,9 @@
....
$ cat .git/reftable/tables.list
-00000001-00000001.log
-00000002-00000002.ref
-00000003-00000003.ref
+00000001-00000001-RANDOM1.log
+00000002-00000002-RANDOM2.ref
+00000003-00000003-RANDOM3.ref
....
Readers must read `$GIT_DIR/reftable/tables.list` to determine which
@@ -940,7 +934,7 @@
3. Select `update_index` to be most recent file's
`max_update_index + 1`.
4. Prepare temp reftable `tmp_XXXXXX`, including log entries.
-5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}.ref`.
+5. Rename `tmp_XXXXXX` to `${update_index}-${update_index}-${random}.ref`.
6. Copy `tables.list` to `tables.list.lock`, appending file from (5).
7. Rename `tables.list.lock` to `tables.list`.
@@ -993,7 +987,7 @@
should always be the case, assuming that other processes are adhering to
the locking protocol.
7. Rename `${min_update_index}-${max_update_index}_XXXXXX` to
-`${min_update_index}-${max_update_index}.ref`.
+`${min_update_index}-${max_update_index}-${random}.ref`.
8. Write the new stack to `tables.list.lock`, replacing `B` and `C`
with the file from (4).
9. Rename `tables.list.lock` to `tables.list`.
@@ -1005,6 +999,22 @@
Each reftable (compacted or not) is uniquely identified by its name, so
open reftables can be cached by their name.
+Windows
+^^^^^^^
+
+On windows, and other systems that do not allow deleting or renaming to open
+files, compaction may succeed, but other readers may prevent obsolete tables
+from being deleted.
+
+On these platforms, the following strategy can be followed: on closing a
+reftable stack, reload `tables.list`, and delete any tables no longer mentioned
+in `tables.list`.
+
+Irregular program exit may still leave about unused files. In this case, a
+cleanup operation can read `tables.list`, note its modification timestamp, and
+delete any unreferenced `*.ref` files that are older.
+
+
Alternatives considered
~~~~~~~~~~~~~~~~~~~~~~~
